1. Field of the Invention
The present invention relates to high-frequency modules used in, for example, antenna front ends of wireless communication devices.
2. Description of the Prior Art
An antenna front end of a wireless communication device may be formed as a high-frequency module in which a switch IC, a SAW filter (SAW duplexer), a matching circuit and so forth are provided in a multilayer substrate (refer to, for example, Japanese Unexamined Patent Application Publication No. 2011-77723).
FIG. 10 is a circuit diagram for explaining an example of a high-frequency module of the prior art.
A high-frequency module 101 illustrated in FIG. 10 includes a switch IC 102, an antenna matching circuit 103, a transmission filter circuit 104, a transmission filter circuit 105, a SAW duplexer 106, a matching circuit 107, a SAW duplexer 108 and a matching circuit 109.
The high-frequency module 101 transmits and receives transmission signals and reception signals corresponding to a plurality of communication systems by commonly using a single antenna. The switch IC 102 has an antenna connection port PsA and connection switching ports PsS1, PsS2, PsS3 and PsS4. The switch IC 102 performs switching to connect any of the connection switching ports PsS1, PsS2, PsS3 and PsS4 to the antenna connection port PsA. The antenna matching circuit 103 is connected to the switch IC 102 via the antenna connection port PsA and performs impedance matching between the antenna and the switch IC 102. The transmission filter circuit 104 is connected to the switch IC 102 via the connection switching port PsS1 and allows a signal of a transmission band of a certain communication system to pass therethrough. The transmission filter circuit 105 is connected to the switch IC 102 via the connection switching port PsS2 and allows a signal of a transmission band of a certain communication system to pass therethrough. The SAW duplexer 106 is connected to the switch IC 102 via the connection switching port PsS3 and is composed of two SAW filters having different pass bands to each other. The matching circuit 107 is connected between the SAW duplexer 106 and the switch IC 102 and performs impedance matching in a band corresponding to the SAW duplexer 106. The SAW duplexer 108 is connected to the switch IC 102 via the connection switching port PsS4 and is composed of two SAW filters having different pass bands to each other. The matching circuit 109 is connected between the SAW duplexer 108 and the switch IC 102 and performs impedance matching in a band corresponding to the SAW duplexer 108. The matching circuit 107 and the matching circuit 109 are each formed of a single inductor. Each of these inductors is connected in parallel with the respective signal line and has one end thereof connected to ground.
In the case where a matching circuit is formed by connecting an inductor in parallel with a signal line as in the above-described high-frequency module, the SAW duplexer exhibits a high-pass attenuation characteristic in which attenuation in a band lower than the pass band is comparatively large and attenuation in a band higher than the pass band is comparatively small when seen from the switch IC side. Consequently, sometimes a harmonic of a reception signal cannot be sufficiently attenuated and intermodulation distortion is generated by the reception signal and the harmonic in the SAW duplexer and a circuit connected downstream of the SAW duplexer.
Therefore, giving the SAW duplexer a low-pass attenuation characteristic in which attenuation in a band higher than the pass band is comparatively large and attenuation in a band lower than the pass band is comparatively small when seen from the switch IC side has been considered. Accordingly, the matching circuit may have a configuration in which a capacitor is connected in parallel with a signal line. However, in such a case, when the capacitor is formed by providing a pair of coupling electrodes, it is necessary to secure a space in which to arrange the pair of coupling electrodes inside a multilayer substrate and as a result, the size of the substrate has sometimes been increased.
In addition, for example, the inductor and capacitor forming the matching circuit and a wiring line that is connected to the matching circuit are formed as inner layer electrodes in the multilayer substrate, and degradation of isolation characteristics and generation of intermodulation distortion has sometimes occurred due to the inner layer electrodes that form the matching circuit coupling with an inner layer electrode connected to another signal line.
Moreover, the wiring line connected to the matching circuit cannot be lengthened without limit due to the generation of loss, degradation of the Q value and also the generation of a signal phase shift in the wiring line. Consequently, when designing the matching circuit, it has been necessary to decide the arrangement and values of other circuit elements with consideration of the length and arrangement of the wiring line, and as a result, there have been large restrictions imposed on the design and the design work has been difficult.